1. Field of the Invention
The present invention relates to a brushless direct current (BLDC) motor, and more particularly to an apparatus for detecting a rotor position in a BLDC motor.
2. Background of the Related Art
A BLDC motor has higher efficiency and is easier to be controlled than other motors. In this respect, the BLDC motor is generally used for variable speed driving of a compressor for refrigerator/air-conditioner and a washing machine.
To drive the BLDC motor, a flux of a stator should be controlled to be electrically orthogonal or have an angle with respect to a flux of a permanent flux from a rotor. To this end, a speed of the motor is controlled in such a manner that a position of a rotor is always detected and switching states of inverter switching elements are adjusted to determine a flux position of a stator in accordance with the detected rotor position. Accordingly, the configuration for detecting the rotor position is essential for the speed control of the BLDC motor.
Furthermore, a driving mode of the BLDC motor is divided into a rectangular current waveform and a sinusoidal current waveform depending on applied phase current.
A related art apparatus for detecting a rotor position of a BLDC motor will be described with reference to the accompanying drawings.
FIG. 1 shows a speed control device of a related art BLDC motor. Referring to FIG. 1, the speed control device includes a BLDC motor 1, a rotor position detector 2, a speed detector 3, a subtracter 4, a speed controller 5, a voltage controller 6, and an inverter 7. The rotor position detector 2 detects a phase of a voltage applied to the inverter 7, which is output to a BLDC motor, so as to detect the rotor position. The speed detector 3 detects a driving speed of the BLDC motor 1 by a signal detected by the rotor position detector 2. The subtracter 4 subtracts a speed detecting signal input from the speed detector 3 from a speed command input from a driving controller (not shown) to obtain a speed error. The speed controller 5 outputs a voltage command (voltage size) of the speed error output from the subtracter 4. The voltage controller 6 determines switching time of the inverter 7 by the signal from the rotor position controller 2 and the voltage command from the speed controller 5. The inverter 7 outputs direct current as a voltage of a variable frequency to correspond to the switching time determined by the voltage controller 6. The BLDC motor 1 is driven by the voltage from the inverter 7.
The operation of the aforementioned related art speed control device of the BLDC motor will be described below.
In a two-phase conducting mode in which current occurs only in a period of 120xc2x0, a speed command output from a driving controller (not shown) and a speed detecting signal xcfx89xcex3 detected by the speed detector 3 are subtracted from the subtracter 4, and the resultant value, i.e., the speed error is output to the speed controller 5.
The speed detecting signal xcfx89xcex3 from the speed detector 3 is calculated by the output of the rotor position detector 2. An encoder or a hall sensor is used as the rotor position detector 2. The rotor position detector 2 the rotor position of the BLDC motor 1 and outputs the detected value to the speed detector 3 and the voltage controller 6.
Subsequently, the speed controller 5 outputs the voltage signal (voltage command), which corresponds to the speed error output from the subtracter 4, to the voltage controller 6.
The voltage controller 6 determines switching state of the inverter 7 in accordance with the voltage signal output from the speed controller 5 to control the speed.
At this time, the encoder or the hall sensor may be used as the rotor position detector 2. In case of the compressor for refrigerator/air-conditioner, it is difficult to use the sensor due to environmental factors such as temperature and pressure. Accordingly, it is necessary to the rotor position from a voltage or current applied to the motor.
In the driving waveforms, the rectangular current waveform permits the rotor position to be detected from a phase voltage in a region to which voltage or current is not applied. In other words, the rotor position can be detected per electrical angle of 60xc2x0 by detecting the point where the phase voltage of the open phase to which voltage or current is not applied becomes zero.
However, in the sinusoidal current waveform, voltage or current is always applied to three phases of A, B and C regardless of the rotor position. Accordingly, in the same manner as the rectangular current waveform, the rotor position cannot be detected with the voltage information of the open phase. Thus, the rotor position can be detected by detecting all the voltages or current of the three phases of A, B and C.
As shown in FIG. 3, the rotor position detector 2 for detecting the rotor position without using the sensor includes a current detector 51, a voltage detector 52, a first position detector 53, a second position detector 54, and a third position detector 55. The current detector 51 detects current of each phase from a predetermined alternating current voltage of each phase output from the inverter 7. The voltage detector 52 detects a voltage of each phase from a predetermined alternating current voltage of each phase output from the inverter. The first position detector 53 detects the rotor position in accordance with the current of the phase A detected by the current detector 51 and the voltage of the phase A detected by the voltage detector 52. The second position detector 54 detects the rotor position in accordance with the current of the phase B detected by the current detector 51 and the voltage of the phase B detected by the voltage detector 52. The third position detector 55 detects the rotor position in accordance with the current of the phase C detected by the current detector 51 and the voltage of the phase CA detected by the voltage detector 52.
Each of the first to third position detectors 53, 54 and 55 includes a first integrator 71 for integrating current of a virtual neutral point for each phase output from the current detector 51, a second integrator 72 for integrating a voltage of a virtual neutral point for each phase output from the voltage detector 52, a mixer 73 for mixing a signal output from the second integrator 72, an operation signal of a signal output from the first integrator 71 and a proportional coefficient R, and an operation signal of the current of one phase among the phases and a proportional coefficient L with one another, and a comparator 74 for comparing a signal output from the mixer 73 with a predetermined signal and outputting the resultant value.
The operation of the aforementioned rotor position detector 2 will be described below.
The predetermined alternating current voltage as shown in FIG. 2 is supplied from the inverter 7 to the respective phases A, B and C of the stator of the BLDC motor. A predetermined power is generated in the BLDC motor 1 in accordance with the predetermined alternating current voltage for each phase output from the inverter 7.
The rotor position detector 2 detects the rotor position of the BLDC motor 1 in accordance with the predetermined alternating current voltage for each phase output from the inverter 7 and outputs the resultant signal.
In other words, the current detector 51 within the rotor position detector 15 detects current for each phase from the predetermined alternating current voltage for each phase output from the inverter 7, and the voltage detector 52 detects the voltage for each phase from the predetermined alternating current voltage for each phase output from the inverter 7.
The first position detector 53 detects the rotor position in accordance with the current for the phase A detected by the current detector 51 and the voltage for the phase A detected by the voltage detector 52. That is, the first integrator 71 within the first position detector 53 integrates the current for the virtual neutral point of each phase output from the current detector 51. The second integrator 72 integrates the voltage for the virtual neutral point of each phase output from the voltage detector 52. The mixer 73 mixes the signal output from the second integrator 72, the operation signal of the signal output from the first integrator 71 and the proportional coefficient R, and the operation signal of the current of the phase A and the proportional coefficient L, and outputs the resultant signal. The comparator 74 compares the signal output from the mixer 73 with the predetermined signal and outputs the resultant signal.
The second position detector 54 detects the rotor position in accordance with the current for the phase B detected by the current detector 51 and the voltage for the phase B detected by the voltage detector 52. That is, the first integrator 71 within the second position detector 54 integrates the current of each phase output from the current detector 51. The second integrator 72 integrates the voltage of each phase output from the voltage detector 52. The mixer 73 mixes the operation signal of the signal output from the first integrator 71 and the proportional coefficient R, the signal output from the second integrator 72, and the operation signal of the current of the phase B and the proportional coefficient L, and outputs the resultant signal. The comparator 74 compares the signal output from the mixer 73 with the predetermined signal and outputs the resultant signal.
Also, the third position detector 55 detects the rotor position in accordance with the current for the phase C detected by the current detector 51 and the voltage for the phase C detected by the voltage detector 52. That is, the first integrator 71 within the third position detector 55 integrates the current of each phase output from the current detector 51. The second integrator 72 integrates the voltage of each phase output from the voltage detector 52. The mixer 73 mixes the operation signal of the signal output from the first integrator 71 and the proportional coefficient R, the signal output from the second integrator 72, and the operation signal of the current of the phase C and the proportional coefficient L, and outputs the resultant signal. The comparator 74 compares the signal output from the mixer 73 with the predetermined signal and outputs the resultant signal.
The principle for detecting the rotor position of the BLDC motor will be modeled as the following equation.
[equation 1]      V    abc    =            R      ·              i        abc              +          L      ⁢              xe2x80x83            ⁢                        ⅆ                      i            abc                                    ⅆ          t                      +          e      abc      xe2x80x83Vabc=[Van Vbn Vcn]Teabc=[ea eb ec]Tiabc=[ia ib ic]Txe2x80x83xe2x80x83[equation 1]
In the above equation, Vabc represents each phase voltage of three phases A, B and C for the neutral point, iabc represents a phase reluctance matrix of three phases A, B and C, R represents a phase reluctance matrix for the three phases A, B and C, L represents a phase inductance matrix for the three phases A, B and C, and eabc represents a voltage in each phase.
In a permanent magnet surface adhesive type BLDC motor in which a permanent magnet is attached to a core of a rotor, reluctance is not changed depending on the rotor position. Accordingly, La=Lb=Lc is obtained.
In the above equation, the phase voltage eabc is proportional to the speed of the motor as shown in the following equation 2 and is a function which is converted to a sinusoidal wave in accordance with the rotor position, as shown in FIG. 4.
ea=ke xcfx89xcex3 cos (xcex8xcex3)xe2x80x83xe2x80x83[equation 2]
            e      b        =                  k        e            ⁢              xe2x80x83            ⁢              ω        γ            ⁢              xe2x80x83            ⁢      cos      ⁢              xe2x80x83            ⁢              (                              θ            γ                    -                                    2              ⁢                              xe2x80x83                            ⁢              π                        3                          )                        e      c        =                  k        e            ⁢              xe2x80x83            ⁢              ω        γ            ⁢              xe2x80x83            ⁢      cos      ⁢              xe2x80x83            ⁢              (                              θ            γ                    +                                    2              ⁢                              xe2x80x83                            ⁢              π                        3                          )            
Since the voltage eabc includes rotor position information, the rotor position can be detected from the information of the voltage.
However, in case where the current of the motor is controlled by the sinusoidal wave, the voltage eabc cannot be detected. Accordingly, the voltage should be inferred from the equation 1. In the equation 1, the voltage can be expressed as the following equation 3 but it is difficult to realize the equation 3 in a circuit or algorithm because a differential term of the current exists in the equation 3. Thus, the voltage is obtained by integrating both sides as shown in the following equation 4.
[equation 3]      e    abc    =            V      abc        -          R      ·              i        abc              -          L      ⁢              xe2x80x83            ⁢                        ⅆ                      i            abc                                    ⅆ          t                    
[equation 4]  "AutoLeftMatch"                                          ∫                                          e                abc                            ⁢                              xe2x80x83                            ⁢                              ⅆ                t                                              =                      ∫                                          (                                                      v                    abc                                    -                                      R                    ·                                          i                      abc                                                        -                                      L                    ⁢                                          xe2x80x83                                        ⁢                                                                  ⅆ                                                  i                          abc                                                                                            ⅆ                        t                                                                                            )                            ⁢                              xe2x80x83                            ⁢                              ⅆ                t                                                                                  =                                    ∫                                                v                  abc                                ⁢                                  ⅆ                  t                                                      -                          R              ⁢                              ∫                                                      i                    abc                                    ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                    t                                                                        -                          L              ⁢                              xe2x80x83                            ⁢                              i                abc                                                                                  =                                    x              abc                        ⁢                          xe2x80x83                        ⁢                          (              t              )                                          
Here, an integrated value of the voltage can be expressed from the equation 2 to the following equation 5. Accordingly, the rotor position can be detected from the integrated value of the voltage obtained by the equation 4.
[equation 5]  "AutoLeftMatch"                                                        x              abc                        ⁢                          xe2x80x83                        ⁢                          (              t              )                                =                      ∫                                          e                abc                            ⁢                              xe2x80x83                            ⁢                              ⅆ                t                                                                                  =                                    [                                                k                  e                                ⁢                                  xe2x80x83                                ⁢                sin                ⁢                                  xe2x80x83                                ⁢                                  (                                      θ                    γ                                    )                                ⁢                                  xe2x80x83                                ⁢                                  k                  e                                ⁢                                  xe2x80x83                                ⁢                sin                ⁢                                  xe2x80x83                                ⁢                                  (                                      θ                    -                                                                  2                        ⁢                                                  xe2x80x83                                                ⁢                        π                                            3                                                        )                                ⁢                                  xe2x80x83                                ⁢                                  k                  e                                ⁢                                  xe2x80x83                                ⁢                sin                ⁢                                  xe2x80x83                                ⁢                                  (                                                            θ                      γ                                        +                                                                  2                        ⁢                                                  xe2x80x83                                                ⁢                        π                                            3                                                        )                                            ⁢                              xe2x80x83                            ]                        T                              
As described above, the rotor position detector 2 detects the point 0 from the signal of the voltage integrated for each phase of the phases A, B and C, so that the rotor position can be detected.
However, the related art apparatus for detecting a rotor position in a BLDC motor has several problems.
To detect the rotor position, the voltages and the current for the phases A, B and C should be detected. In this case, additional equipments for detecting the phase voltages and phase current, such as a resistor and an insulator, are required, thereby increasing the production cost.
Furthermore, devices such as an integrating circuit and a comparator are required to obtain integrated values for the voltages and current signals of the phases A, B and C. This also increases the production cost and an area occupied by the devices.
Accordingly, the present invention is directed to an apparatus for detecting a rotor position in a BLDC motor that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an apparatus for detecting a rotor position in a BLDC motor, in which a rotor position can be detected by only a virtual neutral point voltage and current of one phase in a motor.
Another object of the present invention is to provide an apparatus for detecting a rotor position in a BLDC motor, in which the number of circuits required for the rotor position detection is reduced to save the production cost and reduce an area occupied by a position detecting circuit.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an apparatus for detecting a rotor position in a BLDC motor includes: a first integrator for integrating a signal input through a virtual neutral point of power supply lines of three phases connected to the BLDC motor; a second integrator for integrating a signal input to the power supply line of one phase among the power supply lines of the three phases; an operation unit for mixing a signal output from the first integrator, an operation signal of a signal output from the second integrator and a proportional coefficient R, and an operation signal of a proportional coefficient L and current of the one phase; a first compararison unit for comparing a signal of the one phase output from the operation unit with a predetermined first reference value; a second comparison unit for comparing the signal of the one phase output from the operation unit with predetermined second and third reference values; and a position signal calculator for calculating a position signal by operating signals output from the first and second comparison units.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.